1. Field of the Invention
This invention relates to methods of and apparatus for encoding and decoding component digital video signals.
2. Description of the Prior Art
There has been considerable interest in video signal coding schemes which compress the overall bandwidth of a transmitted video signal. For example, in the 4:2:2 CCIR 601 coding standard, a bandwidth of 13.5 MHz is divided into 6.75 MHz for the luminance component and 3 375 MHz for each of the color components. In order to provide a half-rate system, in other words one with half of the previous overall bandwidth, it has been proposed to reduce this standard to 2:1:1, which halves the available bandwidths for the luminance and color components. Although such reduced bandwidth color components can provide reasonable quality, there is insufficient bandwidth for the luminance component since the Nyquist frequency will be 3.375 MHz and it has been found that at least 5 MHz and preferably more is necessary for good quality reproduction. Various other methods for compressing the video signal into a reduced bandwidth have been proposed, but none has been fully satisfactory since each proposed method has resulted in some disadvantageous effect.
In the conventional PAL and NTSC coding systems, if the CCIR 601 sampling standard of 13.5 MHz for the luminance signal is to be applied, it is found that although both systems can be encoded and decoded fully in the digital domain, neither of the systems is optimised since the sub-carrier relationship to sampling frequency is not simple and the color components are not optimally located within the 6.75 MHz Nyquist bandwidth.
A further factor to be taken into account is that any coding scheme for compressing video signals should be compatible with, or result in minimal adaptation of, existing ancillary equipment. For example, in a digital video tape recorder (DVTR), it should be possible to manipulate the carrier phase for editing operations and for slow motion and shuttle replay.